As the big-box retailer of the online world, it's no surprise that Amazon sees a future in big boxes.

This week, Amazon launched Prime Pantry, a delivery service that allows customers to fill up a single box of non-perishable household goods and have it shipped to them at a flat rate.

In a dig at stores like Costco and Walmart, where customers save by buying in bulk, Pantry users will be able to purchase items like cereal, canned soup, pet food, paper towels and detergent in "everyday sizes," but save by having them shipped in bulk.

Customers can order up to 45 pounds of items, but each box, no matter the size of the shipment (it shows the percentage being filled in the shopping cart), will be delivered for $5.99.

The service is only available to Amazon Prime subscribers, but the flat rate comes on top of the annual $99 membership fee (up from $79 as of April 17).

While Pantry delivers to customers in the contiguous U.S. (not Hawaii or Alaska), the company's other offering in the grocery space, same day service AmazonFresh is currently accessible in three West Coast markets – Los Angeles, San Francisco and Seattle.